1. Field of the Invention
This invention relates to a radio communication apparatus and a radio communication method in which a symbol to be transmitted is generated at every timing for the detection of a received symbol.
2. Description of the Related Art
In a known radio communication apparatus using digital modulation and demodulation, a DSP (digital signal processor) detects every received symbol, and a timer logic outside of the DSP generates a symbol clock signal having a frequency equal to a symbol reception rate and feeds the generated symbol clock signal to the DSP as an external interrupt signal. The DSP repetitively sets a symbol timing in response to the external interrupt signal. At the symbol timing, the DSP derives the value of a received symbol from detection-result data generated through detection and analog-to-digital conversion of a received analog signal. The DSP detects, from the detection-result data, timing-error information indicating the difference of the symbol timing from the Nyquist point. The DSP notifies the external timer logic of the timing-error information. The external timer logic temporally adjusts the external interrupt signal in response to the timing-error information so that the symbol timing set by the DSP will be controlled to reduce the difference of the symbol timing from the Nyquist point. Therefore, the symbol timing is adaptively controlled to follow the Nyquist point or coincide therewith. Even in the event that the symbol reception rate changes, the controlled symbol timing enables the correct value of a received symbol to be derived. Such a known radio communication apparatus is disclosed in, for example, Japanese patent application publication number 2008-160181.
In the known radio communication apparatus, an external-signal-responsive interrupt for setting a symbol timing repetitively occurs in the DSP at a rate equal to the symbol reception rate. In a hypothetical case, every symbol to be transmitted is generated at a timing defined by the symbol timing responsive to the external interrupt signal while the known radio communication apparatus performs transmission. In such a hypothetical case, a proper symbol transmission rate is available.
It is assumed that such a known radio communication apparatus is used for a radio-communication base station designed to simultaneously perform reception and transmission to implement, for example, a relay action or a repeater action. In this assumed case, the following problem may occur.
As the symbol reception rate changes, the symbol timing responsive to the external interrupt signal varies so that the symbol transmission rate varies also. In general, the symbol transmission rate about a signal transmitted from a base station is used as a reference for operation of terminal devices communicating with the base station. Accordingly, the symbol transmission rate is required to be highly accurate and stable. A variation in the symbol transmission rate is undesirable. When the symbol timing is fixed to hold the symbol transmission rate constant, the symbol timing can not follow a change in the symbol reception rate so that an error may occur in the derived value of a received symbol. Such an error means a reduction of the receiving performances of the radio communication apparatus. To solve this problem, it is conceivable to provide external timer logics for reception and transmission respectively. As compared to the presence of only one external timer logic, two external timer logics increase the apparatus cost and the consumed electric power and occupy two interrupt ports of the DSP.